Reference is also made to commonly-owned copending application Ser. No. 09/404,184, filed Sep. 24, 1999; Ser. No. 09/451,319, filed Nov. 30, 1999; Ser. No. 09/456,781, filed Dec. 8, 1999; Ser. No. 09/506,911, filed Feb. 18, 2000; and Ser. No. 09/663,948, filed Sep. 18, 2000. Reference is also made to commonly-owned copending application Ser. No. 09/296,499, filed Apr. 22, 1999.
Bromine-based biocides have proven biocidal advantages over chlorination-dechlorination for the microbiological control of cooling waters and disinfection of waste treatment systems. The water treatment industry recognizes these advantages to be cost-effective control at higher pH values, almost no loss in biocidal activity in the presence of ammonia, and effective control of bacteria, algae and mollusks.
A common way of introducing bromine based biocides into a water system is through the use of aqueous NaBr in conjunction with NaOCl bleach. The user feeds both materials to a common point whereupon the NaOCl oxidizes the bromide ion to HOBr/OBrxe2x8ax96. This activated solution is then introduced directly into the water system to be treated. The feeding of the two liquids in this fashion is necessary because the HOBr/OBTrxe2x8ax96 mixture is unstable and has to be generated on-site just prior to its introduction to the water. Furthermore, the feeding, and metering of two liquids is cumbersome, especially as the system has to be designed to allow time for the activation of bromide ion to occur. Consequently many biocides users have expressed the need for a single-feed, bromine-based biocides. Molecular bromine chloride is deemed to meet these demands. It is a liquid at room temperature and can be fed directly to the water system, where immediate hydrolysis occurs to yield HOBr.
BrCl+H2Oxe2x86x92HOBr+HCl
Bromine chloride is a fuming, red liquid or gas, with a boiling point of 5xc2x0 C., and a vapor pressure of 1800 mm at 25xc2x0 C. It corrodes most metals in the presence of water.
It can be seen that certain characteristics of bromine chloridexe2x80x94especially its corrosiveness, high vapor pressure and fuming tendenciesxe2x80x94necessitate care and skill in its handling and use.
An economically acceptable way of stabilizing high concentrations of aqueous solutions of bromine chloride is described in U.S. Pat. No. 5,141,652 to Moore, et al. The solution is prepared from bromine chloride, water and a halide salt or hydrohalic acid. These solutions were found to decompose at a rate of less than 30% per year and in cases of high halide salt concentration, less than 5% per year. Moreover, solutions containing the equivalent of 15% elemental bromine could be prepared. Unfortunately, the relatively high acidity of these solutions and their tendency to be corrosive and fuming impose limitations on their commercial acceptance.
U.S. Pat. No. 6,068,861, referred to at the outset, describes, inter alia, a new process of forming concentrated aqueous solutions of biocidally active bromine and in so doing, provides novel and eminently useful concentrated aqueous biocidal solutions of bromine chloride. Such solutions are formed by a process which comprises mixing (a) bromine chloride with (b) an aqueous solution of alkali metal salt of sulfamic acid (preferably the sodium salt), the resulting solution having a pH of at least about 7, e.g., in the range of 7 to about 13.5. The amounts of (a) and (b) used are such that (i) the content of active bromine in the solution is at least 100,000 ppm (wt/wt) and (ii) the atom ratio of nitrogen to active bromine from (a) and (b) is greater than 0.93, and preferably is greater than 1.
One objective of this invention is to enable a process for producing concentrated aqueous biocidal solutions of bromine chloride wherein at least a portion of the bromine chloride is being continuously produced and used as one of the continuous feeds, to be carried out not only in a commercially-feasible, highly efficient manner on a continuous basis, but in addition, to include efficient automatic continuous process control in such process. Other objectives may appear hereinafter.
In one of its embodiments this invention provides a process of producing a concentrated liquid biocidal composition which process comprises:
a) continuously forming bromine chloride from separate feed streams of bromine and chlorine by maintaining said streams under automatic feed rate control whereby the streams are continuously proportioned to come together to form bromine chloride;
b) forming an aqueous product having an active bromine content of at least 100,000 ppm (wt/wt), a pH of at least 7, and an atom ratio of nitrogen to active bromine greater than 0.93:1 by continuously or intermittently feeding into mixing apparatus separate feed streams of (i) bromine chloride formed in a) and (ii) an aqueous solution of alkali metal salt of sulfamic acid, under automatic feed rate control.
In a preferred embodiment, bromine chloride and said aqueous solution of alkali metal salt of sulfamic acid are intermittently being fed into said mixing apparatus. In another preferred embodiment, bromine chloride and aqueous solution of alkali metal salt of sulfamic acid are continuously being fed into said mixing apparatus, and product is withdrawn from said mixing apparatus at a rate sufficient to enable the continuous feeding to be maintained. Preferably the automatic feed rate controls in a) and b) are under nested cascade ratio flow control.
Since the reaction between the bromine chloride and the alkali metal sulfamate solution is exothermic, it is desirable to control the temperature of this reaction so that the temperature of the aqueous product being formed does not exceed about 50xc2x0 C., and preferably is in the range of about 25 to about 40xc2x0 C., and most preferably is maintained at about 30xc2x0 C. Such control may be accomplished, for example, by promptly passing the effluent from the mixing apparatus through a proximately-disposed heat exchanger to remove excess heat from the effluent. Another way of accomplishing such temperature control is to suitably precool the aqueous alkali metal sulfamate solution before it reaches the mixing device such that the temperature of the effluent stays within the foregoing temperature conditions. Both such methods of temperature control can be utilized, if desired.
Another embodiment of the above process includes, in addition to steps a), b), and c) as described above, forming, either periodically or continuously, the aqueous alkali metal sulfamate solution by reaction between sulfamic acid and alkali metal base such as alkali metal hydroxide in water.
One preferred embodiment of the above processes includes the following concurrent operation, namely, continuously, but alternately, withdrawing from at least one and then from at least one other of at least two reaction vessels, an aqueous solution of alkali metal salt of sulfamic acid at a rate that maintains a continuous stream of (ii) in b), and during the time the solution is being withdrawn from at least one of at least two such reaction vessels, forming additional aqueous solution of alkali metal salt of sulfamic acid in at least one other of such reaction vessels from which solution is not then being withdrawn. In this way, aqueous alkali metal sulfamate solution can be continuously withdrawn from one or more tanks (xe2x80x9cTank(s) Ixe2x80x9d) to serve as a continuous feed of (ii) in b), while forming more of such solution in one or more other tanks (xe2x80x9cTank(s) IIxe2x80x9d), so that when Tank(s) I is/are depleted, the system is switched to Tank(s) II which then serve(s) as the supply for the continuous feed of (ii) until depleted, and by that time more of such solution has been formed in Tank(s) I. Thus by alternating the supply and the production from one tank (or group of tanks) to another tank (or group of tanks) and switching back and forth between the filled tanks as the supply, a continuous feed of the aqueous alkali metal sulfamate solution can be maintained without material interruption.
Another preferred embodiment of the above processes includes the following concurrent operation, namely, continuously withdrawing an aqueous solution of alkali metal salt of sulfamic acid from a circulating inventory of alkali metal salt of sulfamic acid, the withdrawal being at a rate that maintains the stream of (ii) in b), and continuously replenishing the circulating inventory from a supply of such aqueous solution of alkali metal salt of sulfamic acid from a reaction vessel in which aqueous solution of alkali metal salt of sulfamic acid is produced at least periodically in quantity sufficient to at least maintain such circulating inventory. By maintaining a circulating inventory of the aqueous solution of alkali metal salt of sulfamic acid in a pump around circulation loop, only one reaction vessel is required for forming such solution from sulfamic acid, an alkali metal base such as alkali metal hydroxide, and water.
The above and other embodiments and features of this invention will be still further apparent from the ensuing description, the accompanying drawings, and/or the appended claims.